Corrosion resistant tin-free steel and method for producing same

ABSTRACT

A superior corrosion-resistant tin-free steel is produced by vacuum depositing high-purity metallic chromium onto a thoroughly cleaned steel surface at a temperature within the range 500*1,000* F., and thereafter, before the coated steel is cooled, exposing the freshly deposited coating to an oxidizing atmosphere while the steel is at a temperature of 600* to 1,000* F. The resulting vigorous high-temperature oxidation of exposed steel at cracks and pores to magnetite produces a continuous pore-free and crack-free coating which is lustrous and metallic in appearance and provides exceptional resistance to corrosion.

llnited States Patent Inventors Lawrence E. llelwig Hampton Township, Allegheny County;

Leon L. Lewis, Butler, both of Pa.

Appl. No. 878,548

Filed Nov. 20, 1969 Patented Jan. 11, 1972 Assignee United States Steel Corporation CORROSION RESISTANT TIN-FREE STEEL AND METHOD FOR PRODUCING SAME 10 Claims, No Drawings References Cited UNITED STATES PATENTS 11/1969 Wilkinson 148/63 Primary E.xaminerRalph S. Kendall Att0rneyForest C. Sexton ABSTRACT: A superior corrosion-resistant tin-free steel is produced by vacuum depositing high-purity metallic chromium onto a thoroughly cleaned steel surface at a temperature within the range 500l,000 F., and thereafter, before the coated steel is cooled, exposing the freshly deposited coating to an oxidizing atmosphere while the steel is at a temperature of 600 to 1,000" F. The resulting vigorous high-temperature oxidation of exposed steel at cracks and pores to magnetite produces a continuous pore-free and crack-free coating which is lustrous and metallic in appearance and provides exceptional resistance to corrosion.

BACKGROUND OF THE INVENTION The so-called tin-free steels, i.e., plated sheet steel substitutes for tinplate having protective coatings other than tin are well known in the art and are becoming of increasing significance in the container and canning industries. Commercial tin-free steels are usually produced by electroplating a thin layer of chromium onto the steel surface and subsequently or simultaneously forming a passive oxide or conversion coating over the chromium surface. Although metallic chromium by itself is extremely resistant to corrosion, deposited coatings are usually porous and cracked. Hence, the oxide coating is essential to seal the cracks and pores to protect the steel substrate. Crack-free chromium coatings are known in the art, but the plating conditions for producing such crack-free coatings are so inefficient that commercial adaption thereof is not practical.

Although such plating and oxidation processes as described above do provide some degree protective coatings, the processes are somewhat inefficient and not readily adapted to high-speed operation typical of tinplate production. In addition, the prior art electroplated and oxidized or converted coatings frequently have a tendency to turn brown, and are not usually completely free of cracks because the coatings may crack upon cooling after the passivation treatment.

Vacuum deposition of chromium is another well-known plating procedure which produces a good adherent and lustrous coating. Until recently, however, vacuum-deposited chromium coatings have been used almost exclusively for decorative applications rather than corrosion protection because the vapor-deposited coatings are too porous and discontinuous to protect against corrosion, even when oxidized or converted by conventional procedures. When corrosion resistance is required, vacuum-deposited chromium coatings are either deposited on a lacquered steel or are subsequently protected by a lacquer coating. Recently, vacuumdeposited chromium coatings have been used for protective applications on some steels. To be substantially continuous, and at least in part protective, such coatings must be rather thick, usually 500 microinches or more. Even when very thick, however, such chromium coatings exhibit some discontinuity and fine cracks which substantially lessen corrosion protection from more ideal values.

SUMMARY OF THE INVENTION We have discovered that a very thin vacuum-deposited high-purity chromium coating on a clean steel substrate, which is typically very porous and discontinuous, can be made continuous, pore-free and crack-free, providing superior corrosion protection, if the coating is quickly and vigorously oxidized at elevated temperatures immediately after deposition before any other surface oxidation or passivation reactions can occur.

It is, therefore, an object of this invention to provide a process for producing corrosion-resistant steel products, particularly tinplate substitutes, having a thin protective coating of chromium with the pores and cracks therein sealed with a corrosion-resistant oxide.

It is another object of this invention to provide a process for producing tin-free sheet steels having an adherent lustrous coating which will not brown or discolor and is readily adapted to high'speed processing equipment.

It is a further object of this invention to provide a method for improving the corrosion resistance ofa steel having a thin porous and discontinuous vacuum-deposited chromium coating.

it is still another object of this invention to provide a new and improved corrosionresistant sheet steel suitable as a substitute for tinplate.

DESCRIPTION OF THE PREFERRED EMBODIMENT Although any steel may be coated by the process of this invention, the process will find particular utility in the manufacture of corrosion-resistant tin-free sheet products.

Although the vacuum deposition of chromium in accordance with this invention is performed by well-known prior art techniques, it is essential that contamination of the steel surface and chromium coating be minimized. Simply stated, the steel, after an essential thorough surface cleaning, is heated in a high vacuum thereby degassing the steel, and while heated under the high vacuum, the steel is exposed to high-purity chromium vapors which condense on the steel surface.

In preparing a steel strip for vacuum deposition, it is essential that a good cleaning procedure be used to assure that substantially all surface oxides and oils are removed. We prefer to clean the strip with a short wash in a chelated alkaline solution followed by a one second dip in a 5 percent citric acid pickling solution at F. As an alternative, we have used an extended one minute cathodic cleaning procedure in a chelated 6 percent alkaline solution at F. and a current density of 65 a.s.f. After pickling by any process, the sheet should of course be rinsed, preferably with alcohol or inhibited water to minimize reoxidation. After the strip is cleaned, other forms of reoxidation or surface contaminations should be avoided. It is, therefore, preferable that vacuum depositing follow the cleaning procedure as soon as possible.

To commence vacuum deposition, the steel strip is heated to a temperature within the range 500-l,000 F., and preferably 600 to 800 F., in a vacuum of less than 5X10 torr, and preferably less than lXlO torr, and preferably less than 1X10 Heating the steel to the desired temperature under vacuum will be sufficient to effect suitable degassing thereof, and hence no dwell time is required. After the desired temperature is reached, the strip is exposed to highpurity chromium vapors or a vapor beam in accordance with any of the various prior art practices. For example, elemental or alloyed chromium in solid form will sublime upon heating with an electron beam or resistance heated conical tungsten wire basket. The chromium vapors thus formed will condense on the cooler steel surface.

The vacuum-deposited chromium coating should be applied at a preferred thickness of from I to 3 microinches. Although any thickness within the range 0.2 to 20 microinches or more will provide some benefit, the l to 3-microinch coatings are optimal. Coatings less than about 1 microinch are some what inferior in the final corrosion protection, while coatings thicker than about 3 microinches utilize excessive chromium without improvement.

As would be expected, the exceptionally thin coatings produced as described above would be quite porous and somewhat discontinuous, especially on unpolished surfaces such as that of the cold-rolled steel strip. By itself therefore, the vacuum-deposited chromium coating would afford little protection against corrosion. The crux ofthis invention, therefore, resides in the following oxidation step whereby the thin vacuum-deposited chromium coating is made continuous and protective to the steel substrate. That is, after the desired vacuum deposition of chromium is complete, the freshly coated strip, while still at elevated temperatures of 600 to 1,000 F. is quickly exposed to an oxidizing atmosphere such as air, as for example by exiting the strip from the vacuum chamber. The hot steel exposed at cracks and pores is thereby vigorously and rapidly oxidized to a more complete state of oxidation than is possible at lower temperatures. Upon cooling, the oxidized chromium coating is lustrous and metallic in appearance and provides the steel substrate with exceptional protection against corrosion. Besides the exceptional corrosion protection, the oxidized coating is still extremely thin by prior art standards, being from 1.0 to 5.0 microinches in thickness and weighing no more than 85 mg. per square foot.

If the porous and discontinuous coating is cooled in the vacuum and then subsequently oxidized by prior art practices,

i.e., reheating the coated steel from room temperature in an oxidizing atmosphere, surface oxidation does occur, but it is not nearly as effective in corrosion protection or resistance to discoloration, even if oxidizing temperatures in excess of 600 F. are eventually reached. Hence, the superior product of this invention can be achieved only if high-temperature oxidation, i.e., 600 to l,000 F., is effected with the substantial exclusion of high-temperature oxidation below 600 F.

We refer to the undesired oxidation as high-temperature oxidation below 600 F., because we have learned that a modest degree of room temperature oxidation is not particularly harmful, probably because it is not very extensive. Specifically, coated but unoxidized steels have been cooled to room temperature in the vacuum and then stored in air for periods of about a week or more. These samples, when reheated in a high vacuum to temperatures within the essential oxidizing range, i.e., 600 to 1,000 F., and then quickly exposed to air, did yield the superior protective coating of this invention. On the other hand, samples first exposed to hightemperature oxidation below 600 F. were substantially inferior and could not be improved by further oxidation at temperatures exceeding 600 F. It is apparent therefore that the improved coating of this invention can be achieved only if substantially all oxidation is effected quickly at temperatures within the range 600 to 1,000 F., and that any preliminary oxidation at temperatures below 600 F. down to about 200 F. should be positively avoided.

Although we do not completely understand the mechanism of this invention, we do believe the superior characteristics of our coating results from the combination of the two essential requirements, i.e., the high-purity nature of the chromium coating and the high-temperature oxidation of exposed steel at pores and cracks to form iron oxide magnetite. More specifically, the high-temperature oxidation of this invention causes the pores and cracks in the chromium coating to be plugged with the impervious iron oxide magnetite, Fe O in contrast, lower temperature oxidation typical of the prior art will result in the formation ofa more porous oxide, Fe O The magnetite obviously provides superior corrosion protection. Subsequently, when the oxidized coating is cooled, the high-purity nature of the chromium coating contributes ductility thereto, and hence resists cracking that might otherwise occur at temperatures below 600 F. Hence, the finished product coating contains only the original cracks and pores which are plugged with the impervious magnetite. On the other hand, if the chromium coating should contain more typical impurities, particularly iron oxides at the interface, the chromium coating will be more brittle and will crack upon cooling. The new cracks will then be plugged with the more porous Fe- O formed at temperatures below 600 F.

The chromium coating produced in accordance with this invention is substantially cleaner and of higher purity than most deposition at temperatures exceeding 500 F. prevents absorption of residual gases on the steel surface where they could react with the deposited chromium. In addition, the deposition pressure, using no greater than 4X10 torr, reduces the amount of gas reacting with, or occluded in, the chromium coating as it is deposited. Upon the subsequent high-temperature oxidation, the outer surface of the chromium coating is oxidized to some degree, but not sufficient to alter the more ductile characteristics of the chromium beneath the outer oxide skin.

To aid in a fuller understanding ofthis invention, the following detailed example exemplifies one procedure that may be followed in the practice ofthis invention.

EXAMPLE 1 A steel substrate panel (6X4 inches) was cathodically cleaned in a 6 percent solution of Oakite Rustripper (a proprietary chelating alkaline cleaner) at about 180 F. for 10 seconds at amperes per square foot (a.s.f.), rinsed in tap water, pickled 3 seconds in a 5 percent citric acid solution at about F. or for 1 second at F., rinsed with methanol, warm air dried and placed in the vacuum chamber. The chamber was evacuated to about 5X10 torr, the substrate was heated to 800 F., a measured quantity of chromium was evaporated and condensed on the substrate with a coating weight of 17 mg./sq. ft., and the vacuum chamber was immediately opened to the air and reached atmospheric pressure in less than 10 seconds. The coated panel was then tested.

EXAMPLE 2 A steel panel was cathodically cleaned in 1.5 percent Pennsalt 78 cleaner (a proprietary alkaline cleaner) at about F. for 10 seconds at 65 a.s.f., rinsed in water, pickled 30 seconds in 5 percent citric acid at room temperature, rinsed in fresh citric acid solution, rinsed cathodically for 1 second at about 10 a.s.f. in a solution containing approximately 2 percent sodium dichromate with 0.2 percent chromic acid, rinsed in water, warm air dried, and placed in the vacuum chamber. The chamber was evacuated to less than 10 torr, the substrate was heated to 1,000 F., nichrome (80 percent nickel- 20 percent chromium) was evaporated and condensed on the substrate to reach a coating weight of about 34 mg./sq. ft., and oxygen was rapidly admitted to the vacuum chamber to l atmosphere pressure. The coated panel was then tested.

EXAMPLE 3 A steel substrate panel was cathodically cleaned at approximately 100 a.s.f. at 180 to F. for 1 minute in a 6 percent solution of Oakite Super Rustripper (a proprietary chelated alkaline cleaner), rinsed with a water solution containing Par- From table 1 it can readily be seen that the resulting corro- STORAGE RUST RESISTANCE Rust resistance in storage at 100 F., 85% relative humidity temperaturc Specimen Area rust covered,

during temperature percent coating during Coating Time to deposition, oxidation thickness, At 8 At 1!) 30% rusted.

F. in air, F. microinches days days days 200 0.5 20 10 on. 8 400 0.5 ca. 8 600 0.6 ('11. 8 800 0. 5 50 200 0. 5 8 400 0. 5 8 600 0.6 ca. 8 800 0. 5 10 200 2.0 19 400 2. 0 1'.) (100 2. 0 50 800 2. (1 01 200 2. 0 8 100 .3. 0 8 600 2. 0 8 S00 2. 0 -25 prior art coatings because ofthe extensive cleaning ofthe substrate steel surface prior to deposition. Furthermore, vacuum sion resistance severely drops off as oxidation temperature is reduced.

kin Chemical Company Rust Preventative Additive, warm air dried, and placed in the vacuum chamber. The chamber was evacuated to about 5 l0 torr, the substrate was heated to 600 F., chromium was evaporated from a zircorium-chromium alloy to achieve a coating weight of about 85 mg./sq. ft., the vacuum chamber was quickly opened to a humid air atmosphere, and the panel was removed for testing.

To further exemplify the characteristics of this invention, table 1 below shows 16 typical experimental samples of steel vapor deposited with chromium and oxidized using various parameters. Both parameters within and outside of the scope of this invention are shown for the purpose of exemplifying the importance of high-temperature oxidation.

Table ll below contrasts the corrosion-resistance characteristics (Double Seam Rust Test) of five typical experimental samples processed in accordance with this invention with commercial samples of prior art tin-free steels.

1,000 F. while said steel is subjected to a vacuum of less than about 4Xl0' torr;

c. vacuum-depositing metallic chromium onto the heated steel surface to provide a chromium layer of at least about 0.2 microinches;

d. avoiding any substantial oxidation at temperatures between 200 and 600 F. until after step (e);

e. exposing the chromium coated steel to an oxidizing atmosphere while said steel is at a temperature of from 600 to l,00O F. to form Fe Q, over the steel surfaces exposed through the chromium coating; and

f. cooling said coated steel.

2. The method of claim 1 in which said chromium layer is vacuum-deposited to a thickness of from 0.2 to microinches.

3. The method of claim 1 in which said chromium layer is vacuum-deposited to a thickness of from i to 5 microinches.

TABLE II.VACUUM-DEPOSI'IED-CHROMIUM COATED STEEL CON- TRASTED TO COMMERCIAL PRIOR ART TIN-FREE STEEL IN DOUBLE SEAM RUST TEST 1 Temperature Busting on cover Coating of deposition hook radius, percent thickness, of coating,

Product tested microinchcs F? Average Range Vnc. dep. Cr 0. 5 500 15 9-20 I) 0. 5 800 6 5-8 Do. 2. 0 500 5 3-8 Do 2. 0 800 6 243 D 2.0 800 4 2-6 Prior Art -0. 5 200 41 -53 D -0. 5 200 34 18-54 Do. 0. 5 200 56 28-99 D0 1. 5 200 21 16-44 Do 1. 5 200 26 6-36 Do l. 5 200 18 6-31 D0. -0. 5 200 27 11-62 1 Double Seam Rust test simulates rusting which can occur on edge of can end during pasteurization or food processing. Eleetroplated TFS-CT has rarely done well in this test.

2 Electroplated TFS-C'I point of the aqueous solutions from which they are are well under 200 F.

3 Chromium'ehromium oxide 4 Phosphatechromate coating.

coatings cannot be applied at temperatures over the boiling deposited. In practice, temperatures coating, commercially produced by a one step process.

5 Chromium-chromium oxide coating, commercially produced by a tvm step process.

Our extensive testing and experimentation revealed that the benefits of this invention could not be realized on electrodeposited chromium surfaces. in fact, heating of electrodeposited surfaces in an oxidizing environment actually reduced their corrosion resistance and caused some discoloration.

It should be apparent that various modifications and alternative procedures could be incorporated into the above detailed procedure without departing from the basic concept of this invention. For example, the steel substrate need not be a sheet product intended as a substitute for tinplate. Any cleaning procedure for cleaning the surface of the substrate may be suitable if it is effective in removing substantially all surface scale and oil. In addition, it should be apparent that one of the prime requirements is that the steel, once vacuumdeposited with metallic chromium, not be subjected to lowtemperature oxidation. Therefore, the freshly deposited chromium coating need not be oxidized as described immediately after deposition, provided there is no high-temperature oxidation below 600 F.

We claim:

1. A method of producing a corrosion-resistant steel by depositing a protective oxide coating thereon, the steps comprising:

a. cleaning the surface of the steel substrate to remove substantially all scale and oils;

b. heating the cleaned steelto a temperature of from 500 to 4. The method of claim 1 in which said vacuum is less than about 1X 1 0 torr.

5. The method of claim 1 in which said vacuum deposition of chromium is effected at a temperature of from 600 to 800 F.

6. The method of claim 1 in which said chromium coated steel is exposed to said oxidizing atmosphere immediately after the vacuum deposition.

7. The method of claim 1 further comprising the steps of cooling the chromium coated steel to temperatures below about 200 F. in said vacuum after the vacuum deposition of chromium, storing the cooled coated steel in air for periods no greater than about 1 week, reheating the coated steel in a vacuum of less than about 4 lO torr to a temperature of from 600 to l,000 F., exposing the reheated coated steel to an oxidizing atmosphere, and then cooling the coated steel.

8. A steel having a corrosion-resistant protective coating consisting ofa substantially oxygen free steel substrate having a high-purity vacuum-deposited chromium coating of at least 0.2 microinches in thickness, all pores, cracks and discontinuities in said chromium coating being plugged with a dense, nonporous magnetite.

9. The steel of claim 8 in which said chromium coating has a thickness of from 0.2 to 20 microinches.

10. The steel of claim 8 in which said chromium coating has a thickness of from i to 5 microinches.

UNITED STATES PATENT OFFICE ETIFKQATE or Patent NO. 3 63u,1 ;7 Dated January 11, 1972 lnventofls) Lawrence E Helwig et a1 It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Colu n 2, lines 31 and 32, cancel "and preferably less than 1X10 torNand insert a. period (L); line 47, "some what." should read somewhat Column 8-, the para raph placed immediately before and continued after "Table l should be inserted in column 5, between lines 13 and 1 4,

Signed and sealed this 7th day of November 1.972.

(SEAL) Attest:

EDWARD MELETCHERJR. ROBERT GOTTSCHALK Attsting Qfficm Commissioner of Patents USCOMM-DC 60376-P69 FORM PO-1050 (10-69) 1* us, covzmmsur PRINTING omcs i969 o-see-aaa. 

2. The method of claim 1 in which said chromium layer is vacuum-deposited to a thickness of from 0.2 to 20 microinches.
 3. The method of claim 1 in which said chromium layer is vacuum-deposited to a thickness of from 1 to 5 microinches.
 4. The method of claim 1 in which said vacuum is less than about 1 X 10 4 torr.
 5. The method of claim 1 in which said vacuum deposition of chromium is effected at a temperature of from 600* to 800* F.
 6. The method of claim 1 in which said chromium coated steel is exposed to said oxidizing atmosphere immediately after the vacuum deposition.
 7. The method of claim 1 further comprising the steps of cooling the chromium coated steel to temperatures below about 200* F. in said vacuum after the vacuum deposition of chromium, storing The cooled coated steel in air for periods no greater than about 1 week, reheating the coated steel in a vacuum of less than about 4 X 10 4 torr to a temperature of from 600* to 1,000* F., exposing the reheated coated steel to an oxidizing atmosphere, and then cooling the coated steel.
 8. A steel having a corrosion-resistant protective coating consisting of a substantially oxygen free steel substrate having a high-purity vacuum-deposited chromium coating of at least 0.2 microinches in thickness, all pores, cracks and discontinuities in said chromium coating being plugged with a dense, nonporous magnetite.
 9. The steel of claim 8 in which said chromium coating has a thickness of from 0.2 to 20 microinches.
 10. The steel of claim 8 in which said chromium coating has a thickness of from 1 to 5 microinches. 